The use of a viscous fluid clutch for controlling the rotation of an engine cooling fan in an automotive vehicle is known in the art. Prior to the use of a viscous fluid clutch and when fuel efficiency was not a significant concern, an engine cooling fan was coupled directly to a water pump shaft. The cooling fan was operative so long as the engine was running. In these prior configurations, the engine cooling fan speed was proportional to the engine speed. That is, the cooling fan spun faster as the engine speed increased. However, such continuous fan operation is unnecessary and somewhat wasteful. While it is needed at idle and slower vehicle speeds, the engine cooling fan becomes less important when a vehicle is traveling at higher speeds because outside air is forced through the radiator without the aid of the fan.
This realization, and the need for greater fuel efficiency, led to the development cooling fan viscous clutches. The viscous clutch allows the fan blades to essentially freewheel when the extra cooling assistance provided by the fan is unnecessary. This feature has resulted in enhanced fuel economy and the reduction of unnecessary noise generated in the engine compartment. Specifically, many vehicles employ a multi-blade cooling fan that is secured to the viscous fan clutch. The viscous clutch is installed between an accessory pulley (typically a water pump pulley) of the vehicle and its radiator. The viscous clutch is designed to drive the fan at high speeds during desired operating modes, which can approach the input speed of the viscous clutch. This may approach the rotational speed of the engine.
The operation of the viscous clutch may be controlled as a function of the engine's operating temperature. In these thermostatically controlled arrangements, the clutch operates to drive the fan approaching engine speeds when cooling is required. On the other hand, the viscous clutch permits the fan to operate at low speeds when cooling is not required. Such thermostatic control of the fan through the clutch reduces airflow noise caused by fan rotation and load on the engine. This results in increased horsepower and improved fuel economy.
Known thermostatically controlled viscous clutch systems rely on the temperature of the air passing through the radiator or the temperature of the cooling water of the engine to switch the on/off control for the fan. While these prior art viscous fan clutches have met with wide acceptance with automotive and truck manufacturers, more accurate control of the viscous fan clutch offers additional advantages. Complex engine control management systems have been incorporated into most automotive and truck vehicles to control emissions, fuel economy as well as other engine operating characteristics. Because they provide improved monitoring of numerous operating parameters of the engine, it is desirable to utilize these engine management control systems to control operation of the viscous clutch as well.
However, the use of engine management control systems to control viscous clutch operation has led to certain problems. In particular, most viscous coupling fan drives are not engaged or maintained in a working state when the vehicle is in an off state. In the off state, much of the viscous coupling fluid is evacuated from the torque transfer or working chamber. However, an insufficient volume of fluid in the working chamber may result in an improper response of the clutch during vehicle start-up conditions. This is because the shear rate of the viscous fluid is dramatically increased such that the residual fluid in the working gap is unable to behave in a viscous manner. In this “stall mode” or under-speed condition, the fluid cannot sufficiently transfer torque to the clutch output and to the fan to meet the cooling needs of the system. This condition may persist even though the engine control management system provides signals to actuate the clutch at its upper limit. In some instances, the under-speed problem will persist for several minutes or longer unless input conditions of the vehicle changed.
Thus, it would be advantageous to have a viscous fan clutch that achieves proper response characteristics, even during start-up conditions or when other operating conditions are present.